Since the late 1940's when hydraulic fracturing of the formation was first practised, it has been found necessary to inject solid granular propping agents into such formations to prevent closure of the fractures. Sand (flint) was apparently the first such solid propping agent to be employed. The McGuire patent, U.S. Pat. No. 2,950,247 suggests the use of aluminum oxide spheres as proppant agents. Bark's U.S. Pat. No. 3,890,072 suggests the use of sintered bauxite spheres for oil well proppants. Cooke's patent, U.S. Pat. No. 4,068,718 teaches the use of sintered bauxite particles made according to the teaching of U.S. Pat. Nos. 3,079,243 or 3,421,492 to Ueltz for oil well proppants.
Cooke's U.S. Pat. No. 4,068,718 referred to above, requires a density of at least 3.4 g/cc in the proppant in order to provide sufficient compressive strength. However, the sintered bauxite proppant described in the prior art, though excellent in permeability (measured by a standardized permeability test--see Cooke U.S. Pat. No. 4,068,718) and in chemical resistance (measured by a standardized API acid solubility test), are not cost effective for intermediate depth wells of pressures between 5,000 to 10,000 psi because of higher cost of bauxite as raw materials. Further, a low density proppant with density less than 3.4 g/cc is easier to pump into the well and may be carried more readily into the cracks in the formation than a more dense proppant. A low density proppant will also additionally benefit the user as lower amounts of proppant will be required to fill a given volume of the well formation. Colpoy's U.S. Pat. No. 3,976,138 suggests "alumina" proppants of a wide range density and alumina content but contains no teachings of how the pellets are made and what kind of raw materials were used for the invention and also shows the results only for material with a density of 3.4 g/cc or greater. Fitzgibbon, U.S. Pat. No. 4,427,068 describes a method of manufacturing a lower density (less than 3.4 g/cc) proppant by blending diaspore, barley and flint clays with varying amounts of higher cost bauxite raw material. Results in the Fitzgibbon U.S. Pat. No. 4,427,068 suggest that as the amount of bauxite is decreased the proppant permeability (i.e. quality) dimishes and to keep the reduction of permeability to about 50%, when the closure stress is increased from 2,000 psi to 10,000 psi the amount of bauxite required to be blended to the diaspore clay is about 40%. This increases the cost of the raw materials substantially.
Further the acid solubility (measured by a standardized API method) of the proppants made according to Fitzgibbon U.S. Pat. No. 4,427,068 ranged between 4.86 to 7.4% which is approximately 50 to 150% higher than that for sintered bauxite proppants described in Cooke's U.S. Pat. No. 4,068,718 and are now commercially available. The need for blending bauxite to various clays in Fitzgibbon U.S. Pat. No. 4,427,068 also adds additional processing steps and the complexity of co-milling of the two various raw materials of different chemical characteristics to a fine size in a manufacturing process. All the additional processing steps and use of bauxite blend in Fitzgibbon U.S. Pat. No. 4,427,068 add cost to the manufacture of the proppant and the cost/productivity justification of using a low cost lower density proppant material in intermediate depth gas and oil wells (5,000 feet to 15,000 feet) may be greatly reduced.
Bakker U.S. Pat. No. 3,642,505 teaches making mullite aggregate for refractory purposes, by grinding a siliceous bauxite (13 to 14% SiO.sub.2), forming pellets under pressure, then calcining, and subsequently firing the pellets to at least 1535.degree. C.
The proppants after firing should be generally spherical (Krumbein number of at least 0.8) and have a size range of from 12, U.S. Standard Sieve Size, to 70 U.S. Standard Sieve size. The preferred size for most applications is through mesh 20 on mesh 40. U.S. Standard Sieves have mesh openings as follows:
______________________________________ Size Inches Millimeters ______________________________________ 12 0.0661 1.68 20 0.0331 0.841 25 0.0278 0.707 30 0.0234 0.595 40 0.0165 0.420 70 0.0083 0.210 ______________________________________